Internal inlet/outlet

I read in the manual(Section 10.31.1) on the subroutine USRSRC that if a user wishes to prescribe the velocity at some internal point in the domain, one way of proceeding is to use an internal inlet and outlet.

Can someone explain what this means ?

Does it mean that for a cell one face is defined as an inlet and the other as an outlet ?

You can create a solid region and put an inlet (into the flow domain) and an outlet (pulling from flow domain) on either side of the solid.

My preferential method of fixing a velocity (and this is also in the manual) is to put a momentum source in a fluid cell (or USER3D patch) such that SU=C*Uset and SP=-C. This will give a source term which is equivalent to

S = SU + SP(U) = C*Uset - C*U = C * (Uset-U)

Uset is the directional velocity you wish to fix.

C is an arbitrary constant, however, the larger it is, the stronger the source and more chance for over shoots and convergence problems. Really small C values will not influence the flow enough and take a long time to converge.

Note that SP is automatically multiplied by U. This will tend to fix the U value to Uset over time.